If you want to make pixelshit, just use your own engine. Or Godot.
If you want to make something 3D, you're kind of outta luck as far as potato-friendly and ACTIVE engines go. I think Unity's system requirements kinda exceed the limits of a shitty PC these days unless you want to spend extra effort tweaking the lighting and shaders. Godot's 3D workflow ain't quite ready. Source may work, but it's no longer supported very well and its workflow is largely hated by most people because of its proprietary filetypes. Also, it's a $50,000 license if you want to sell your game.
You could use Irrlicht if you're okay with using C++. That engine runs just fine on my shitty old laptop that struggles to run Half-Life 2 on max settings.